As far as I know, you can NOT just go active duty. That's the short answer, what follows is a very long story that I will try to keep down to the basic facts.
As a DCO, you are commissioned a reserve officer in the USNR with a drilling obligation as a selected reservist. This means one weekend a month, two weeks a year. You can apply for active duty orders subsequent to your initial training being done. However, you are still a SELRES officer on active duty. Orders come in a variety of flavors, <90 days, >90 && <180, or >180 days. >180 are rare, and tough to come by.
There is also the possibility of being mobilized as a Title 10 recall, or a Presidential Selected Reserve Callup. In the end though, when the orders end, you go back to one weekend/two weeks.
More than likely this all makes no sense to someone who is not currently SELRES, but a RESERVE RECRUITER (notice I did not say active duty recruiter; they are different) can clear up your questions.
Also, check out
http://kellybeamsley.com/military this site is made by a DCO Intel type and he keeps up with the latest changes to the program.
I don't want to discourage you, but I also don't want you to get into something that you won't be happy with in the end.
As to the age waiver question, yes, age waivers are fairly common, it seems especially in INTEL. Best thing to do is submit a package and find out.
Length of service school varies from community to community but I'm almost sure that they all fit into one or two 2 week period. In other words, it may take you two years, and two 2 week periods, but you will get the same training as a new Ensign who is active duty.
Hope that helps!
R/
CTT2